Titanium alloy based dispersion-strengthened composites

ABSTRACT

Titanium based metal matrix composites reinforced with ceramic particulate are well known, based on a blend of titanium alloy powders with ceramic powders, e.g., aluminum oxide powders, utilizing a low energy ball milling process, followed by cold compacting and sintering to produce an appropriate composite. This prior art process is disadvantaged from the point of view that there are virtually no particles in the blend below the micrometer size range, which lack has a deleterious effect on the subsequent processing of the composite. This problem has been overcome by utilizing dry high energy intensive milling in the process, which has the effect of providing the necessary number of small particles below the micrometer size range as well as enhancing the reactivity of different particles with one another. In order to produce a titanium base alloy alumina metal matrix composite, titanium dioxide powder is blended with aluminum powder and subjected to dry high energy intensive milling until the separate particle phases achieve a size of 500 nanometers maximum. The intermediate powder product is then heated to form the titanium alloy/amumina metal matrix composite in which the ceramic particles have an average diameter of no more than 3 μ, and the oxide consists of more than 10% and less than 60% by volume fraction of the total composite. The composites have extensive application to tough and strong engineering alloys.

TECHNICAL FIELD

The present invention is directed to the preparation of a metal matrixcomposite reinforced with fine oxide particulate, and in particular atitanium alloy/alumina composite, and to a method of manufacture of suchcomposites

BACKGROUND ART

The use of composite materials formed from fine fragments of desiredmaterials is well known. The uses of these materials are known, thoughnew applications are continually being found. However, the technology isrelatively new and there are significant gaps in the prior art.

For instance, while many composite blends are known, many areas stillremain to be explored and experimented with. Similarly, the techniquesand methods of preparing composites and their pre-cursors are alsoincomplete, despite being relatively well established in some areas.Consequently, one object of the present invention is to extend the rangeof knowledge within this field, as well as attempting to increase thenumber of choices to users of the technology.

Metal Matrix Composites (MMCs) are composites of a tough conventionalengineering alloy and a high strength second phase material, which maybe an oxide, nitride, carbide or intermetallic. Oxide DispersionStrengthened (ODS) alloys come at one end of the spectrum of MMCs. Theseare composites of a tough engineering alloy and a fine dispersion of anoxide. Typically, in order to obtain the required dispersion, there mustbe no more than 10% volume fraction of the oxide second phase, which mayhave a size of 10's of nm. At the other end of the MMC spectrum are theCERMETS in which the “second phase” exceeds 50% of the volume fraction,i.e. the oxide, carbide, nitride or intermetallic, in fact, forms theprimary phase and the metal is the secondary phase.

Titanium alloy metal matrix composites reinforced with ceramicparticulate are known, though traditionally these are usually producedby using conventional and known powder metallurgy techniques. In theknown powder metallurgy routes, titanium alloy powder is blended withceramic powders such as aluminium oxide powders. This blending isusually performed using a low energy ball milling process. The powdermixture is then cold compacted and sintered to produce bulk titaniumalloy matrix composite.

However there are several disadvantages associated with the prior art.Firstly, it is a requirement that the titanium or titanium alloy powdersare prepared according to a separate and known method. This can berelatively expensive and must be performed independently of thecomposite forming process. In contrast, ceramic powders are readilyavailable so this does not represent a problem for the prior art.However, the range of available particle sizes of the ceramic powdersdoes represent a problem. Typically, economic manufacturing processes ofthe ceramic powders is limited in that the smallest readily availablepowders are in the micrometer size range. While this is adequate formost composites, it is now recognised that smaller sized ceramicparticles, or proportions of smaller sized ceramic particles, canimprove the physical and mechanical characteristics of the compositeproduct. By way of example, this is now well known in concretetechnology which uses exceptionally finely sized silica fume particlesto increase the overall strength and durability of the resultingcement/concrete matrix.

U.S. Pat. No. 5,328,501 (McCormick) discloses a process for theproduction of metal products by subjecting a mixture of one or morereducible metal compound with one or more reducing agent to mechanicalactivation. The products produced are metals, alloys or ceramicmaterials which this specification states may be produced as ultra-fineparticles having a grain size of one micron or less. A variety ofspecific reactions are given by way of example, but in all cases, themethod is dependent on the mechanical process producing the requiredreduction reaction. Furthermore, the patent is not directed towards theproduction of metal matrix composites reinforced with fine ceramicparticulate.

There is no disclosure of titanium/alumina composites, nor of anymethods for producing such composites.

There are some significant limitations in the prior art which increasesthe expense of producing composite materials, and which also limits thephysical and mechanical characteristics of the composite product.

It is a further object of the present invention to address the foregoingproblems or at least to provide the public with a useful choice.

DISCLOSURE OF INVENTION

According to one aspect of the present invention, there is provided amethod of producing a metal matrix composite including high energymilling of a mixture of at least one metal oxide with at least one metalreducing agent in an inert environment to produce an intermediate powderproduct substantially each particle of which includes a fine mixture ofthe metal oxide(s) and the reducing metal(s) phases, and heating theintermediate powder product to form the metal matrix compositesubstantially each particle of which includes an alloy matrix of themetal(s) resulting from reduction of the metal oxide(s) reinforced withfine metal oxide particles resulting from oxidation of the metalreducing agent(s).

According to a further aspect of the present invention, there isprovided a method of producing a titanium alloy/alumina metal matrixcomposite from titanium oxide and aluminium including high energymilling of a mixture of titanium oxide with aluminium in an inertenvironment to produce an intermediate powder product substantially eachparticle of which includes a fine mixture of titanium oxide andaluminium phases, and heating the intermediate powder product to formthe titanium alloy/alumina metal matrix composite substantially eachparticle of which includes titanium alloy matrix reinforced with finealumina particles.

The invention also provides for metal matrix composites and, inparticular, titanium/alumina metal matrix composites produced inaccordance with these methods, and also for consolidated products formedfrom such composites.

According to a further aspect of the invention, there is provided ametal matrix composite including a first phase metal or metal alloy anda second phase metal oxide in fine particulate form, the particleshaving an average diameter of no more than 3 μm, and the metal oxidecomprising more than 10% and less than 60% volume fraction of thecomposite.

Other aspects of the invention may become apparent from the followingdescription which is given by way of example only.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical micrograph showing the microstructure of eachparticle of the intermediate powder produced by high energy ball millingof TiO2/Al powder mixture for 8 hours. The white phase is Al and thedark phase is TiO2. (Magnification 1500×).

FIG. 2 is an optical micrograph showing the microstructure of eachparticle of the powder produced after heat treating the intermediatepowder product for 4 hours at 700° C. The white phase is titanium alloyand the dark phase is alumina. (Magnification 1500×).

DETAILED DESCRIPTION OF INVENTION

In the following description the invention is described in relation to aprocess for the manufacture of a titanium alloy/alumina metal matrixcomposite. However, it should be appreciated that the invention is morebroadly directed towards a particular method of manufacturing metalmatrix composites using high energy milling and subsequent heattreatment, and the invention is not limited to composites of titaniumalloy and aluminium oxide.

The process of the invention can broadly be sub-divided into two steps.In the first step, the milling operation, powders of the metal oxide(for example TiO₂) and a metal reducing agent (for example aluminium)are together subjected to high energy milling in order to produce aparticulate material in which each particle comprises a mixture of veryfine phases of the metal oxide and the metal reducing agent, preferablythe phases have a size of no more than 500 nanometers. The secondprinciple step involves heating this intermediate powder product toproduce a reduction reaction and phase change resulting in a metalmatrix composite in which each particle comprises a mixture of very finephases of the reduced metal alloy (e.g. titanium or titanium/aluminiumalloy) and an oxide or oxides of the reducing metal (e.g. alumina). Inthis final composite the oxide phases may have sizes in the range 20nanometers to 3 microns.

With the selected reactants, and under the conditions prescribed, thehigh energy milling process produces the required particlecharacteristics with very little or no substantial reduction. With themix of very fine phases in the particles of the intermediate powder, thereduction that occurs during heating results in a composite withbeneficial physical and mechanical characteristics.

With reference to the production of a titanium alloy/alumina composite,the overall process involves the production of a composite powderconsisting of titanium metal, or a titanium alloy (which is intended toinclude titanium metal in its purest form as well as specific alloys)and aluminium oxide. Typically this involves the reaction of titaniumdioxide with aluminium metal in the reaction process:

3TiO₂+4Al→2Al₂O₃+3Ti

If necessary, the oxides of other metals (such as vanadium) may beincluded though typically this is in small or trace amounts. The levelsare at the user's discretion and will depend upon the type of alloymatrix of the material which they intend to produce, or the level ofdoping required in the final matrix. Typically, however, the levels ofother metal oxides will be kept to substantially 8% or lower (byweight).

Further, it has been found in initial trials by the applicant that highpurity reactants, such as often prescribed for composite manufacture,are not necessarily required. High grade ores of titanium (i.e. rutile)may be sufficiently pure to produce acceptable product characteristics.As a general guide, purity levels of substantially 98.5% or greater (byweight) for all of the reactants is sufficient. In some applications,lower purities may be acceptable, though it is envisaged that for mostapplications the purity levels will be kept to substantially 95% orgreater (by weight). User's discretion can be applied, for in someinstances certain impurities may be acceptable in the resulting product.

It is also contemplated that the process to produce a titanium/aluminacomposite may commence with reduction of ilmenite with aluminium as aprecursor step.

The TiO₂ and aluminium components are reacted, not in the method of atypical thermite process, but rather using a combination of high energymilling apparatus and thermal treatment.

In one example, the milling may involve using high energy ball millingapparatus. The energy of the balls should be sufficient to deform,fracture, and cold weld the particles of the charge powders.

While the conditions of the milling process can be varied to achieve thedesired result, typically the balls will be of a suitable material suchas stainless steel and will be typically of a diameter of substantially5-30 mm inclusive. Balls outside of this range may be used. Acombination of balls of different sizes may also be used.

It has been found that a weight ratio between the balls and the powderswhich is substantially within the range 4:1-10:1 (by weight, inclusive)is preferred though once again weight ratios outside of this range maybe chosen at user discretion.

Whilst specific reference is made to the use of high energy ball millingapparatus, it is not intended that the invention be restricted to simplythis type of milling, although the apparatus must involve a high energysystem capable of providing energy sufficient to deform, fracture andcold weld particles. Other apparatus capable of providing the requiredconditions are also contemplated and will be understood by personsskilled in the art. It is also considered that a split discus-type millapparatus may be appropriate. Such apparatus is described in WO 98/17392(Devereuex), the specification and drawings of which are incorporatedherein by reference.

Preferably the milling process is performed under an atmosphere inert tothe components. Preferably this is a noble gas as titanium oxides arereactive to nitrogen under suitable conditions. A mixture of variousinert gases may also be used, with the preferred gas being argon.

The proportion of titanium oxide and aluminium is usually chosen so thatat least the normal stoichiometric ratios are achieved. If, for userrequirements, a percentage of included metal oxides is meant to remain,then the proportion of aluminium may be dropped. Similarly, it may bedesirable to have as one of the products of the process, an impactedTi—Al alloy, in which case the proportion of aluminium metal in thereactant mix will be increased. In practice, it has been found that aweight ratio between titanium oxide and aluminium powders in the range1.8:1—2.3:1 (inclusive) is an acceptable range for most applications.

The components are placed within the milling apparatus and the processis continued until a powder having the desired particle characteristicsis attained. Normally, it is anticipated that the given period Will bein the range of 2-10 hours, although this will depend upon the actualparameters of the system and choices made by the user. Typically, at theend of the milling process there will be a blended powder comprisingfine fragments including a mixture of fine phases, mainly TiO₂ and Al,with substantially a size of less than 500 nanometers.

The intermediate product is then subjected to thermal treatment under aninert atmosphere. Preferably this comprises treatment at a temperaturenot exceeding 750° C., for a period exceeding 30 minutes. Preferably thetemperature is maintained at around 700±50° C. for a period of up to 4hours inclusive. Again these parameters may be altered according to userrequirements and need. However, the selected temperature is importantfor producing a final product with optimal characteristics. Too high atemperature will inhibit the reducing potential of the aluminium. On theother hand, the higher the temperature the greater the titaniumaluminide (Ti₃Al) content, and titanium aluminide may add importantstrength characteristics to the final product.

Typically, after the thermal treatment, each particle of the powderconsists of nanometer-sized alumina (Al₂O₃) particles embedded in amatrix of titanium alloy; although the alumina particle average size mayrange from about 20 nm to 3 μm. Such a composite may be referred to as afine oxide metal matrix composite

A number of additional steps may be employed in the process of thepresent invention to further modify the characteristics and componentsof the metal matrix composite.

In particular, the volume fraction of alumina may be reduced (from about60% to 40% or less) by pre-reduction of the titanium oxide with hydrogenat a temperature of 700° C. or greater. A preferred temperature is about900° C. This pretreatment step results in a powder which includes anumber of daughter oxides with lower oxygen content, titanium hydrideand titanium phases. This is a way of controlling the volume fraction ofalumina in the final composite.

In addition, or alternatively, the alumina volume fraction in the finalproduct may be reduced by adding titanium powder to the mixture oftitanium oxide and aluminium.

By increasing the quantity of aluminium in the initial mixture ofreactants to 20% or more above the stoichiometric ratio for the reaction3TiO₂+4Al→2Al₂O₃+3Ti a higher titanium aluminide (Ti₃Al) content may beachieved in the final composite. The higher the proportion of differenttitanium alloys in the final composite the lower the volume fraction ofalumina and the smaller the size of alumina particles.

With those additional steps the alumina content of the titanium/aluminametal matrix composite can be reduced to below 60% volume fraction andpreferably to the range 20% to 30% volume fraction of the composite, andthe alumina particles tend to be of a smaller size.

The heat-treated titanium/alumina metal matrix composite may be returnedto the mill one or more times to refine the shape of particle andfurther reduce the size of particle. A more regular-shaped particleprovides for preferred characteristics in the final product.

The preferred metal matrix composite produced by a process of thepresent invention has an average particle size for the oxide particles(or second phase) in the range 20 nm to 3 μm, and an average compositeparticle size not greater than 100 μm.

The various steps of the preferred method of the present invention, asoutlined above, may be carried out as distinct sub-processes in separateapparatus, for example, pre-reduction with hydrogen may be performed ina separate furnace, with high energy milling carried out in the mill,and subsequent heat treatment or “annealing” in the same or a differentfurnace. Alternatively, and with appropriate mill apparatus, the wholeoperation may be conducted in the mill.

Solid composite articles may be formed from the composite. Typically thepowder is consolidated using known techniques. Quite simply this maycomprise the use of routine metallurgy processes, such as coldcompacting the powder under an inert atmosphere. It should beappreciated that other techniques for forming composite articles fromblended materials may also be employed.

Some general comments about the present invention include the fact thattitanium metals or alloys prepared by separate processes are notessential; high grade ores comprising oxides of titanium or other metalsmay be employed. This not only avoids separate preparation steps, butalso the purification steps often associated with the other knownmanufacturing processes.

Further the average size of the oxide particles in the compositematerial is typically much finer than can be attained using mostconventional prior art techniques. In the prior art, in order to attainthe fine oxide particle sizes of the present invention, it willgenerally be necessary to further process the reactants prior to theiruse in forming a composite. With such a small size of reinforcementparticles, the titanium alloy composites of the invention potentiallypossess higher fracture toughness than conventional composites.

As a comparison, the prior art prepares titanium alloy metal matrixcomposites by conventional powder metallurgy routes. In this route,preprepared titanium alloy powder is blended with ceramic powder such asaluminium oxide powders using a low energy ball milling process. Thepowder mixture is then cold compacted and sintered to produce bulktitanium alloy matrix composite materials. One limitation of the priorart method is that the average size of the ceramic particles in thematerials prepared this way is normally in the micrometer size range,which is considerably larger than what is attainable according to thepresent invention.

The invention is further described with reference to specific examples,which should not be construed to limit the scope of the invention.

EXAMPLE 1

A ball milling apparatus is used in which the impact energy of the ballsis sufficient to deform, fracture and cold weld the particles of thecharge powders. The charge powders, titanium oxide and aluminiumpowders, and the balls (e.g. stainless steel balls) with a diameter of5-30 mm are placed in a hardened steel container which is sealed underan inert atmosphere (normally argon). The total weight ratio between theballs and the powders is in the range of 4:1-10:1. The weight ratiobetween the titanium oxide and aluminium powders is approximately 2:1

Some excess amount of starting aluminium powder may be needed to adjustthe composition of the titanium alloy in the final product. The sealedcontainer is placed in a commercially available apparatus whichfacilitates high energy ball milling. Through high energy ball millingfor a given period of time in the range of 2-10 hours, a new type ofpowder will form. Each particle of the new powder will be a composite offine fragments.

The raw materials of the process are economical titanium dioxide powder(rutile, TiO₂) with purity not lower than 98.5% in weight, and aluminiumpowder with purity not lower than 98.5% in weight. The average particlesize of the titanium oxide and aluminium powders is not larger than 300μm. The impurities will stay in the final materials, but the detrimentaleffects (if there are any) on the properties will be controlled throughadjusting powder processing parameters.

Raw materials with a high percentage of impurity might be used, but theconsequence is that the properties of the final materials arecompromised.

Vanadium pentoxide powder with a purity not lower than 98.5% can beincluded in the starting materials. The vanadium oxide is reduced by thealuminium through the process, and the metallic vanadium will go intothe titanium alloy matrix of the final composites to improve themechanical properties of the material. The percentage of the vanadiumpentoxide in the starting powder mixture is in the range of 0-8 wt %(percentage by weight). The average particle size of the vanadiumpentoxide is not larger than 300 μm. An example of the raw materials is:

60-67 wt % Titanium oxide powder (rutile, average particle size <300 μm)

31-35 wt % Aluminium powder (average particle size <300 μm)

0-8 wt % Vanadium pentoxide (average particle size <300 μm).

As described above, the product of this high energy ball milling processis a type of homogeneous composite powder each particle of whichconsists of fine fragments of mainly titanium oxide and aluminium and asmall percentage of other oxides or phases. The average particle size isnot larger than 100 μm. The shape of the particles is irregular.

The ball milled powder is then treated thermally under an inertatmosphere at a temperature around 700° C. for a given period of time inthe range of 1-5 hours. After this thermal treatment, each particle ofthe powder consists of mainly nanometer sized Al₂O₃ particles embeddedin a matrix of titanium alloy.

Bulk pieces or shaped components of composite materials may be producedby consolidating the processed powder materials using a routine powdermetallurgy process. The powder metallurgy process may involve coldcompacting the powder and subsequent sintering of the powder compactunder an inert atmosphere.

EXAMPLE 2

A mixture of titanium oxide (TiO₂) and aluminium (Al) powders withTiO₂/Al weight ratio of 1.85:1 was added in a hardened steel container.The titanium oxide/aluminium weight ratio was controlled in such a waythat the amount of aluminium was 20% in excess of the amount ofaluminium required to fully reduce the titanium oxide. A number of steelballs were added to the charge in the container. The size of the ballswas 10 mm in diameter, and the ball/powder weight ratio was 4.25:1.

The container containing the charge was sealed under an argon atmosphereand then put on a ball mill apparatus to facilitate a milling process inwhich the impact energy of the balls was sufficient to deform, fractureand cold weld the particles of the charged powders. After the powdercharge had been milled in this way for 8 hours, an intermediate powderproduct had been produced. Substantially each particle of the powderincluded a mixture of titanium oxide and aluminium phases with a sizeless than 500 nm, as shown in FIG. 1.

The intermediate powder product from the ball milling process was thenheat treated at a temperature of 700° C. for 4 hours under an argonatmosphere. Heat treatment resulted in a powder of titanium alloy matrixcomposite reinforced by alumina particles with an average particle sizein the range of 100 nm-3 μm, as shown in FIG. 2. Due to the excessiveamount of aluminium, the matrix was mainly Ti₃Al phase. The volumefraction of alumina particles in the composite was approximately 57%.

EXAMPLE 3

The titanium oxide (TiO₂) powder was heat treated in a furnace under aflow hydrogen atmosphere at 900° C. for 4 hours. Through thispre-reduction step, the TiO₂ was partially reduced to a mixture ofTi₇O₁₃, TiO and other titanium oxides with various oxygen contents. Inthis way, the total oxygen content in the titanium oxide powder wasreduced to a lower level.

A mixture of the hydrogen pre-treated titanium oxide powder andaluminium powder was added in a steel container together with a numberof steel balls. The weight ratio between titanium oxide and aluminiumwas controlled in such a way that the amount of aluminium was sufficientto fully reduce the partially reduced titanium oxides. The ball/powderweight ratio was in the range of 4:1-10:1 and the size of the balls wasin the range of 5-30 mm. The container was sealed under an argonatmosphere and put on a ball mill apparatus to facilitate a millingprocess in which the impact energy of the balls was sufficient todeform, fracture and cold weld the particles of the charged powders.After the powder charge had been milled in this way for a time in therange of 2-10 hours, an intermediate powder product had been produced.Substantially each particle of the powder included a mixture of titaniumoxide and aluminium phases with a size less than 500 nm.

The intermediate powder product from the ball milling process was heattreated at a temperature of 700° C. for 4 hours under an argonatmosphere. Heat treatment resulted in a powder of titanium alloy matrixcomposite reinforced by alumina particles with an average particle sizein the range of 20 nm-3 μm. The volume fraction of the alumina particlesin the composite was in the range of 20-50%.

Aspects of the present invention have been described by way of exampleonly and it should be appreciated that modifications and additions maybe made thereto without departing from the scope thereof.

What is claimed is:
 1. A method of producing a metal matrix compositeincluding high energy milling of a mixture of at least one metal oxidewith at least one metal reducing agent in an inert environment toproduce an intermediate powder product substantially each particle ofwhich includes a fine mixture of the metal oxide(s) and the reducingmetal(s) phases, and heating the intermediate powder product to form themetal matrix composite substantially each particle of which includes analloy matrix of the metal(s) resulting from reduction of the metaloxide(s) reinforced with fine metal oxide particles resulting fromoxidation of the metal reducing agent(s).
 2. A method of according toclaim 1 further including a pre-reduction step including exposing the atleast one metal oxide to hydrogen gas at a temperature above 700° C.prior to introduction of the at least one metal reducing agent.
 3. Amethod according to claim 1 wherein substantially each particle of theintermediate powder product includes a fine mixture of the metaloxide(s) and the reducing metal(s) phases with a size of 500 nm or less.4. A method according to claim 1 wherein the metal matrix compositeincludes fine reducing metal oxide particles having an average diameterwithin the range of substantially 20 nanometers to 3 microns inclusive.5. A method according to claim 1 wherein the high energy milling is in ahigh energy ball mill.
 6. A method of producing a titanium alloy/aluminametal matrix composite from titanium oxide and aluminium including highenergy milling of a mixture of titanium oxide with aluminium in an inertenvironment to produce an intermediate powder product substantially eachparticle of which includes a fine mixture of titanium oxide andaluminium phases, and heating the intermediate powder product to formthe titanium alloy/alumina metal matrix composite substantially eachparticle of which includes titanium alloy matrix reinforced with finealumina particles.
 7. A method according to claim 6 wherein in theheating step the intermediate powder product is heated to a temperaturenot exceeding 750° C. for a period exceeding 30 minutes.
 8. A methodaccording to claim 7 wherein the intermediate powder product is heatedto a temperature of substantially 700+/−50° C. for a period ofsubstantially 1 to 6 hours inclusive.
 9. A method according to claim 6further including a pre-reduction step including exposing the titaniumoxide to hydrogen gas at a temperature above 700° C. prior to theintroduction of aluminium.
 10. A method according to claim 6 whereinsubstantially each particle of the intermediate powder product includesa fine mixture of titanium oxide and alumina phases with a size of 500nanometers or less.
 11. A method according to claim 6 wherein the finealumina particles have an average diameter within the range ofsubstantially 20 nanometers to 3 microns inclusive.
 12. A methodaccording to claim 6 wherein the high energy milling is in a high energyball mill.
 13. A method according to claim 12 wherein the balls of theball mill have a diameter between 5 and 30 mm inclusive.
 14. A methodaccording to claim 13 wherein the total weight ratio between the ballsand components being milled (balls:components) is in the range 4:1 to10:1 inclusive.
 15. A method according to claim 6 wherein die highenergy milling is provided by split-discus milling.
 16. A methodaccording to claim 6 wherein the inert atmosphere includes one or moreof the noble gases.
 17. A method according to claim 6 wherein thetemperature and duration of heating during the heating step is adjustedto optimise titanium aluminide content.
 18. A method according to claim6 wherein the titanium oxide is an ore of titanium.
 19. A methodaccording to claim 6 wherein the purity of the titanium oxide ispreferably 98.5% or greater (by weight).
 20. A method according to claim6 wherein the purity of the aluminium is 98.5% or greater (by weight).21. A method according to claim 6 wherein the ratio between titaniumoxide and aluminium in the following reaction is approximatelystoichiometric: 3TiO₂+4Al→2Al₂O₃+3Ti.
 22. A method according to claim 6wherein the quantity of aluminum is substantially 20% higher than astoichiometric ratio for the reaction: 3TiO₂+4Al→2Al₂O₃+3Ti.
 23. Amethod according to claim 6 further including the step of returning thetitanium alloy/alumina metal matrix composite for further high energymilling to refine the particle shape and/or size.
 24. A method accordingto claim 6 wherein oxides of other metals are included with the titaniumoxide.
 25. A method according to claim 24 wherein there is 8% or less ofoxides of other metals.
 26. A method according to claim 25 wherein theother metal oxide or oxides includes another transition metal element.27. A method according to claim 26 wherein the other transition metalelement is vanadium.
 28. A method according to claim 6 wherein the highenergy milling and heating steps are conducted in a common environment.29. A method according to claim 9 wherein the high energy milling,heating and pre-reduction steps are conducted in a common environment.30. A metal matrix composite produced according to the method claim 1.31. A titanium alloy/alumina metal matrix composite produced accordingto the method of claim
 6. 32. A metal matrix composite including a firstphase metal alloy and a second phase metal oxide in fine particulateform, the particles having an average diameter of no more than 3 μm, andthe metal oxide comprising more than 10% and less than 60% volumefraction of the composite.
 33. A metal matrix composite according toclaim 32 wherein the metal oxide comprises 20 to 30% volume fraction ofthe composite.
 34. A titanium alloy/alumina metal matrix compositesubstantially each particle of which includes titanium alloy matrixreinforced with fine alumina particles, the alumina particles comprisingmore than 10% and less than 60% volume fraction of the composite.
 35. Atitanium alloy/alumina metal matrix composite according to claim 34 inwhich the alumina particles have an average diameter of no more than 3μm.